The present invention relates to intraocular lenses and to methods for making such lenses. More particularly, the present invention relates to intraocular lenses having one or more enhanced physical properties, for example, tensile strength and/or flexibility and/or fatigue resistance, which benefit the lenses.
The use of intraocular lenses (IOLs) to improve vision and/or to replace damaged or diseased natural lenses in human eyes, particularly natural lenses impaired by cataracts, has achieved wide acceptance. Accordingly, a variety of IOLs has been developed for surgical implantation in the posterior or anterior chambers of the eye according to a patient's needs.
Known IOLs comprise an optical lens portion or optic which includes an optical zone, and one or more, preferably two, supporting structures, called fixation members or haptics, which are secured to and extend outwardly, in a generally curved direction, from the optic and are for contacting eye tissue to fix or hold the IOL in the proper position after implantation. The optic and the fixation member or members may each comprise a material such as a homopolymer of methyl methacrylate or polymethylmethacrylate (PMMA) or a copolymer containing methyl methacrylate or a polycarbonate or the like. The entire IOL, that is the optic and the fixation member or members, may be made of a single piece or item of material, for example, PMMA.
In order to obtain a high degree of performance, the IOL should be made of a material or materials which have good physical or mechanical properties. For example, the fixation member or members should have sufficient tensile strength, flexibility and fatigue resistance so as not to break or separate from the optic during IOL implantation and to be effective in long term use in the eye. Certain materials, in particular, the methyl methacrylate-containing polymers noted above, have certain properties, such as optic clarity and biocompatibility, which make them very useful in IOLs. However, it would be advantageous to enhance certain other physical properties, as noted above, of such materials to further benefit IOLs made at least in part from such materials.
European Patent Publication No. 0438043A2 discloses IOLs made from PMMA which is subjected to stretching along at least two mutually angulated axes to increase the materials' dimension along the axes of stretch by 20% to 65%, preferably by 40%, thereby providing increased tensile strength and flexibility. Several disadvantages are apparent with such a "multi-axial" stretching system. For example, the material may not be uniformly stretched, which can cause distortions and other irregularities in the material and the final IOL product. In addition, because a relatively complex and cumbersome clamping/stretching system is employed, a relatively high percentage of the stretched material must be discarded. Further, the amount or degree of stretching is quite high, apparently because of the relative inefficiency of the "multi-axial" stretching technique in providing improved mechanical properties. Materials which are highly stretched have a tendency to be more difficult, relative to unstretched materials, to manufacture, for example, machine, into IOLs.
Kataoka U.S. Pat. No. 4,550,057 discloses compressing PMMA sheets to reduce the thickness of the sheet by a factor of at least 3 and increase impact resistance by a factor of at least 10. Large compression forces and/or temperatures on the order of 130.degree. C. to 160.degree. C. are employed to achieve this large degree of compression. These compressed sheets are suitable as a glazing material for windows for vehicles and buildings. This patent does not in any way disclose or suggest anything about IOLs or making IOLs.
Fortin U.S. Pat. No. 3,632,841 discloses compression stretching large acrylic sheets at temperatures of 250.degree. F. and above between polished, heated and lubricated plates to form compressed sheets which are about one-third as thick as the original sheets. This patent discloses that such compression stretching provides improved physical and optical properties. This patent does not in any way disclose or suggest anything about IOLs or making IOLs.
Franz et al U.S. Pat. No. 4,454,203 discloses coating an acrylic substrate with a compatible polymeric film which is less extensible than the substrate and pressing the coated article to reduce thickness so that the compressed article has a thickness of about one-third that of the original article. Reduced amounts of compressive force are apparently required to achieve this large reduction in thickness, and the resulting plastic article is said to have improved optical quality. This patent does not in any way disclose or suggest anything about IOLs or making IOLs.
There continues to be a need for IOLs having enhanced properties and for methods for making such IOLs.